The invention is directed to an apparatus for joining workpiece parts, preferably workpiece parts extending over a large area, along a joint region (in this instance, at least two-dimensional weld) by means of transmission welding.
Generally, to carry out transmission welding two workpiece parts to be connected to one another are put together in such a way as to directly contact along a contact zone. The joint region lies within the contact zone. The first workpiece part is transmissive for the laser beams and faces a laser source. The second workpiece part is absorptive for the laser beams. The laser beams penetrate the first workpiece part, impinge on the upper layers of the second workpiece part, where they are absorbed and are converted into heat energy. The upper layers of the second workpiece part are fused which, due to heat conduction, also brings about a fusing of the adjoining layers of the first workpiece part and a bonding of both workpiece parts to form one workpiece.
If the two workpiece parts are to be joined to one another along a joint region that is not identical to the contact zone, the impingement of the laser beams on the joint region must be limited so that other areas within the contact zone are not also welded together.
One possibility for this consists in the use of a laser beam which is focused in a point or a line on the joint region as is described in patent document EP 1 405 713 B1. An optical lens which focuses the laser beam, is formed as a roller or ball and sits on the surface of the transmissive workpiece is moved along the joint region in synchronization with the laser beam. In so doing the workpiece parts to be welded are pressed together locally by the lens and joined to one another in a precise manner along the joint region. However, joining larger surfaces proves to be very time-consuming.
Patent document EP 997 261 B9 discloses another method by which two workpiece parts are joined to one another in a joint region by transmission welding. In so doing the exposed surface of a transmissive workpiece part is covered by a mask that is opaque to laser light, and laser beams impinge on the mask in the form of a laser curtain which is generated through collimation and focusing of laser beams of one or more laser sources. The mask is perforated in conformity to the joint region so that those areas of the contact zone which are not to be joined to one another are masked. The laser beams blocked by the mask are reflected.
The laser beams impinge on the surface of the absorbing workpiece part along a laser line, this laser line being divided into line segments by the configuration of the mask. If the mask is perforated in conformity to a line segment, the line segment is illuminated by the full power of the laser beams; if the mask is not perforated, the respective line segment is not illuminated. Workpiece parts and laser line are moved relative to one another so that the laser line is guided along the joint region. In so doing the laser line is constantly adapted to the position and extension of the joint region currently being swept over. The extension of the laser curtain and, therefore, the maximum extension of the laser line can be adjusted by changing the working distance between the laser source and mask. Further, the energy density of the laser line can be adapted by changing the output of the laser beams.
It is unfavorable in terms of energy efficiency that a portion of the emitted laser beams is not used and, further, is reflected in a more or less diffuse manner, which increases the thermal load on the apparatus and laser source and necessitates additional measures for protecting the operator. Further, no steps are provided for influencing the power density distribution of the laser beams on the workpiece surface.
An apparatus in which these disadvantages are overcome is disclosed by the Laid Open Application DE 100 07 391 A1. By means of the apparatus disclosed therein influence can be exerted on the power density distribution as well as on the shape of the focal point of a laser beam generated on the workpiece.
For this purpose, a planar, spatially resolving beam modulator for generating a predefinable power density distribution is arranged between laser beam source and workpiece. The spatial resolution achieved by the beam modulator is achieved through a matrix of individually controllable cells which is arranged in a plane, the cells being arranged perpendicular to this plane in the beam path of the laser beam source. Depending on the quantity of cells, a laser beam coming from the laser beam source is divided into individual constituent beams. The transmission of the corresponding constituent beam can be influenced by each of these mutually independent cells. By these means, in the region of the matrix the power density distribution can be selectively adjusted over the channel cross section of the entire laser beam before it strikes the workpiece.
For modulation of the constituent beams, the cells have either a movable micro-mirror or a micro-polarizer or are constructed as liquid crystal cells. With these devices, the transmission of every cell can be adjusted in a continuous manner from maximum transmission to complete blocking of the beam. This affords a wide range of possibilities for sequential or simultaneous working of materials. In an embodiment of the apparatus, for example, a simultaneous working of the workpiece on a surface defined by the beam modulator is carried out in that only those constituent beams required for working a contour arranged two-dimensionally on the workpiece are simultaneously released. The adjustment of a uniform power density distribution along the shape of the entire contour is carried out simultaneously through a gradation of the transmission of the individual enabled cells with respect to one another.
Since the beam modulator is arranged directly in the beam path, the materials used for constructing it must have the highest possible destruction threshold. In order that a laser beam which generally has irregularities in its beam profile that are already close to the destruction threshold at certain points can be prevented from impinging on the beam modulator and in order to increase the dynamic range of the beam modulator, additional steps are needed to homogenize the laser beam even before it impinges on the beam modulator. It can be assumed that the exacting demands imposed on the material properties and on the functional configuration of the beam modulator, the use of additional optics for homogenizing the laser beam striking the beam modulator, and the preparation of a required control represent a significant expenditure for production of the apparatus. Moreover, the principle employed for this purpose can only be meaningfully applied for working small workpieces.
In an apparatus disclosed in Laid Open Application DE 10 2010 007 717 A1, a substantially simpler possibility is described for joining two planar workpiece parts along a structured joint region extending over a large area by means of a line-shaped laser beam source comprising a plurality of individually controllable individual emitters without use of a mask.
The apparatus comprises a carrier (in this case, receptacle) which is designed so that two workpiece parts extending two-dimensionally in X and Y direction can be positioned relative to one another, a line array (in this case, laser beam source comprising a plurality of individually controllable individual emitters, preferably laser beam emitters, collectively forming a line array) which is directed to the receptacle and aligned in X direction, a device for transporting the line array relative to the receptacle in Y direction, and a control for spatially resolved operation of the individual emitters. According to an embodiment example, the line array comprises laser diodes which are arranged adjacent to one another in direction of their slow axis and whose laser beam is collimated in the fast axis direction by a cylindrical lens arranged upstream in the radiating direction corresponding to the Z direction.
The line array which completely spans the joint region in one extension direction generates a laser curtain which executes a relative movement with respect to the workpiece parts and in so doing sweeps over the entire joint region in the second extension direction thereof By means of selectively actuating the individual laser diodes during the relative movement, a corresponding laser power impinges exclusively on the joint region. Zones in which no joint is present are not affected by the laser power and therefore need not be masked.
In this way, it was possible to find a comparatively energy-efficient solution for welding larger workpieces. However, the laser beams impinge on the workpiece parts without being influenced so that the laser beams generally have an inhomogeneous power density distribution in the form of a Gaussian profile. Accordingly, it is impossible to generate sharply contoured joint regions with a homogeneous melt, e.g., in the form of narrow, tight welds. As a rule, a sharply defined joint region is demanded when the joint region is visible to the eye and has a determining influence on the appearance of a workpiece. A homogeneous melt is demanded when the workpiece parts must be joined to one another in a sealing manner.